According to some suggestions that have been made on an elastic boundary wave device, the device has a typical structure as follows: dielectric material is laminated on a surface of a piezoelectric substrate; an interdigital transducer (IDT) and a reflector, which serve as electrodes, are disposed at the boundary between the piezoelectric substrate and the dielectric material; and an acoustical layer is formed on the upper surface of the dielectric material so that an extraneous response in a higher-order mode is suppressed. A conventional elastic boundary wave device shown in FIG. 21 has piezoelectric substrate 21 as a first medium layer, dielectric film 22 as a second medium layer, Au-film 23 as an IDT electrode, and acoustical layer 24. The technique above is disclosed in the pamphlet of International Publication No. 2005/069486.
In the conventional substrate configuration, disposing acoustical layer 24 is effective in suppressing an extraneous response in a higher-order mode. However, the dielectric material of acoustical layer 24 has a substantial thickness of approx. 1λ. The thickness has been an obstacle to suppressing unwanted waves that propagate through the boundary between piezoelectric substrate 21 and dielectric film 22.